How can I identify the ESP used to send a spam email using the email headers?
Michael Ko
Co-founder & CEO, Suped
Published 31 Jul 2025
Updated 18 Aug 2025
6 min read
When a spam email lands in your inbox, it can be frustrating, especially when it appears to come from a reputable brand. Beyond the obvious signs of a suspicious message, understanding where it truly originated can be a powerful tool in your defense. Email headers hold a wealth of hidden information that can help you trace the path of an email and potentially identify the Email Service Provider (ESP) or sending platform behind it.
Identifying the ESP behind a spam email isn't always straightforward. Spammers and phishers often employ techniques to obscure their true origin. However, by dissecting the raw email headers, we can uncover crucial clues about the sending infrastructure, helping us understand the flow and ultimately, the ESP involved.
Understanding email headers
Email headers are like a digital passport, containing a record of every server an email has passed through on its journey to your inbox. They include metadata such as sender and recipient information, timestamps, subject lines, and crucially, technical routing details. While the visible "From" address can easily be spoofed, the underlying headers provide a more authentic trail.
To access these headers, most email clients offer an option like "Show Original," "View Source," or "Show Headers." The exact steps vary by client, but a quick search for your specific email provider and "view email headers" will typically guide you. Once you have the full headers, you're looking for specific fields that reveal the email's journey and the servers involved. For more on how to interpret these, you can refer to resources on reading email headers.
A crucial header for tracing the origin is the Received header. Emails typically accumulate multiple Received lines as they travel from one server to another. They are read from bottom to top, with the bottommost Received entry often pointing to the initial sending server or ESP.
Key headers for ESP identification
Several header fields can provide strong indicators of the ESP. Beyond the "Received" headers, look for proprietary "X-" headers that ESPs often add, such as X-Mailer, X-Provider, X-Campaign-ID, or X-CSA-Complaints. These headers are not standardized but are frequently used by ESPs to add their own identifiers, which can be a direct giveaway. Another useful field is the Message-ID, which often contains a domain name associated with the sending ESP or infrastructure.
The Return-Path (also known as the "Envelope From" or "Bounce Address") also provides clues. This header specifies where bounces and other mail system messages should be sent. In many cases, it points to a domain owned or managed by the ESP, distinct from the visible "From" address that might be spoofed. Examining these specific fields can significantly narrow down the potential ESP. For a deeper dive into identifying the underlying platform, check out our guide on how to determine an email sending platform from headers.
Here's an example of key headers to look for:
Sample email headersplain
Received: from mail-oi1-f49.google.com ([209.85.218.49])
by mx.yourdomain.com with ESMTPS id xyz.123
for <recipient@yourdomain.com>; Mon, 15 Mar 2023 10:00:00 -0400 (EDT)
Received-SPF: pass (google.com: domain of user@example.com designates 209.85.218.49 as permitted sender)
Authentication-Results: mx.yourdomain.com; dmarc=pass (p=none dis=none) header.from=example.com
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com; s=s1;
h=from:to:subject:message-id:date:mime-version;
bh=...
b=...
Return-Path: <bounce-user@example.com>
X-Mailer: Mailchimp
Message-ID: <unique.id@example.com>
Tracing the IP address and leveraging tools
Once you've identified the Received header that appears to be the initial sending server, you'll find an IP address within the square brackets (e.g., [192.0.2.1]). This IP address belongs to the server that sent the email. Performing a reverse DNS (rDNS) lookup on this IP can often reveal the ESP's domain name, as many ESPs configure their IP addresses to resolve to their own specific domains.
However, tracing the IP address isn't always foolproof. Spammers might use compromised servers, proxy networks, or rapidly rotating IP addresses to evade detection. Furthermore, many ESPs use large pools of shared IP addresses, making it difficult to pinpoint a single malicious sender without direct cooperation from the ESP. Despite these challenges, IP tracing remains a vital step in the process of identifying the origin of suspicious emails. For details on how to identify the source of emails from unrecognized IP addresses, consult our guide.
Manual IP lookup
Extracting the IP address from the "Received" header is the first step. You can then use online IP lookup services to find out who owns the IP block and sometimes the associated organization. This can often point directly to an ESP, cloud provider, or hosting company.
Keep in mind that some IP addresses might belong to a large cloud provider, and further investigation is needed to narrow down the actual ESP using that infrastructure.
Automated tools and services
Specialized tools are available that automate the process of parsing email headers and identifying ESPs. These tools analyze various header fields, including proprietary ones, and cross-reference them with known ESP patterns. This can be significantly faster and more accurate than manual analysis for routine checks.
While useful, no single tool can guarantee 100% accuracy, especially with sophisticated spamming techniques.
Advanced methods and challenges
Spammers constantly evolve their methods to bypass detection. They might forge headers, use open relays, or compromise legitimate accounts to send their unsolicited mail. This makes identifying the ultimate ESP a complex task, as the observable headers might only show an intermediate server rather than the true origin. Some services even rewrite headers, adding another layer of complexity, which is why it's important to understand how to identify phishing emails when this occurs.
Even with thorough header analysis, you might only uncover the infrastructure provider (e.g., Amazon Web Services or Microsoft Azure) rather than the specific ESP. In such cases, reporting the abuse to the infrastructure provider is the next best step. They may be able to trace the activity to a specific customer account or take action against the offending party if it violates their terms of service.
Conclusion
Identifying the ESP behind a spam email is a crucial step in understanding the email ecosystem and improving your personal or organizational email security. By carefully analyzing email headers, especially the "Received" and proprietary "X-" headers, you can gather valuable insights into the origin of unsolicited messages. While not always a straightforward process due to the evolving tactics of spammers and phishers, the ability to dissect and understand these headers empowers you to take more informed action.
Ultimately, a combination of manual header analysis, IP tracing, and leveraging specialized tools can significantly enhance your ability to identify and mitigate spam. This knowledge helps not only in reporting abuse effectively but also in recognizing patterns that can inform your spam filtering strategies and protect your inbox more efficiently. Understanding how emails are routed and authenticated helps you stay one step ahead in the fight against unwanted mail.
Views from the trenches
Best practices
Always access the full, raw email headers to get the complete picture of the email's journey and server hops.
Focus on the bottom-most "Received" header, as it usually indicates the first server the email interacted with.
Perform reverse DNS lookups on identified IP addresses; this can reveal the associated domain or organization.
Look for any non-standard "X-" headers, as these often contain ESP-specific identifiers or campaign details.
Common pitfalls
Relying solely on the "From" address in a spam email, as it is easily spoofed and rarely indicates the true sender.
Misinterpreting "Received" headers by reading them from top to bottom, instead of the correct bottom-up order.
Assuming a large cloud provider's IP (e.g., Amazon, Microsoft) is the ultimate sender instead of an underlying ESP.
Not understanding that some legitimate email services rewrite headers, potentially obscuring the original ESP.
Expert tips
Use online tools that parse and interpret email headers automatically to quickly highlight key information and potential ESPs.
Cross-reference domain names found in headers (like Message-ID) with public DNS records to find associated email services.
If an ESP is consistently allowing spam, reporting the full headers to their abuse desk can help them address the issue internally.
Understand that some spam campaigns use highly transient infrastructure, making immediate identification and blocking difficult.
Expert view
Expert from Email Geeks says that analyzing the Received headers is often the most reliable way to trace the email's journey and potential origin, reading them from bottom to top.
March 1, 2023 - Email Geeks
Marketer view
Marketer from Email Geeks says that they've found the Message-ID header useful because it often contains a domain name related to the sender's infrastructure.
